Microtopography effects on pedogenesis in the mudstone-derived soils of the hilly mountainous regions

Topography is a critical factor that determines the characteristics of regional soil formation. Small-scale topographic changes are referred to microtopographies. In hilly mountainous regions, the redistribution of water and soil materials caused by microtopography is the main factor affecting the spatial heterogeneity of soil and the utilization of land resources. In this study, the influence of microtopography on pedogenesis was investigated using soil samples formed from mudstones with lacustrine facies deposition in the middle of the Sichuan Basin. Soil profiles were sampled along the slopes at the summit, shoulder, backslope, footslope, and toeslope positions. The morphological, physicochemical, and geochemical attributes of profiles were analyzed. The results showed that from the summit to the toeslope, soil thickness increased significantly and profile configuration changed from A–C to A–B–C. The total contents of Ca and Na decreased at the summit, backslope, and footslope, while the total contents of Al, Fe and Mg showed an opposite trend. On the summit and shoulder of the hillslope, weathered materials were transported away by gravity and surface erosion, exposing new rocks. As a result, soil development in these areas was relatively weak. In flat areas such as the footslope and toeslope with sufficient water conditions, the addition of weathered components and the prolonged contact between water, soil, and sediment led to further chemical weathering, resulting in highly developed characteristics. Microtopography may influence physicochemical properties, chemical weathering, and redistribution of water and materials, causing variations in pedogenic characteristics at different slope positions.


Study area
The study area, Tongnan District and Dazu District, located in the central Sichuan Basin (Fig. 1).The area is characterized by predominantly hilly topography and a subtropical monsoon climate.Vegetation types in the study area are consist primarily of subtropical evergreen broad-leaved forests.The slope of the study area, calculated using a 12.5 m DEM, ranges from 0° to 69°, with a corresponding slope length factor varying from 0 to 55.64.In the period of 2001 to 2020, the multi-year average precipitation and temperature were 1125.4 mm and 18.1 °C, respectively.According to the third national land resource survey, the predominant land use types in the study area were cultivated land and forest land, both of which account for more than 65% of the total land area (http:// ghzrz yj.cq.gov.cn, http:// www.cqtn.gov.cn).The soils were classified as Cambisols in the WRB classification 28 .

Soil sampling
Five natural and typical toposequences were identified, and along each toposequence, five profiles were selected for sampling based on the microtopography conditions.The sampling points were detailed in Table 1.According to the 1:200,000 regional geological map of China (https:// geocl oud.cgs.gov.cn) and the field survey, the parent material of all the sampling points was purple mudstone of the Upper Jurassic Shaximiao Formation in Mesozoic.Additionally, in order to ensure uniformity, other soil-forming factors such as climate (specifically rainfall and temperature) and organisms (mainly grasses and crops as the predominant surface vegetation, with soil animals detailed in Table 2) were also investigated for each sampling point, in addition to the terrain (Table 1).

Physical and mineralogical methods
The soil bulk density was determined using the soil core (volume = 100 cm 3 ) method.Prior to conducting the soil particle size analysis (PSA), hydrogen peroxide was used to remove organic matter.The removal of iron oxide was achieved using a sodium bicarbonate-buffered, sodium dithionite-citrate system, while sodium acetate was utilized to remove carbonate.Sodium hexametaphosphate was used to disperse soil particles and PSA was determined by pipette method 32 .
Soil porosity (φ) can be calculated by the following formula 32 , where ρ b and ρ p represent bulk density (g cm −3 ) and particle density, respectively.Particle density was determined by pycnometer method.X-ray diffraction (XRD: D8 Advance X-ray diffractometer, Bruker, Germany) was utilized to analyze the mineral composition of soil samples.The soil samples were crushed to particle size < 1 mm, soaked in distilled water, and dispersed by ultrasonic wave.Carbonate and organic matter were removed from the samples, and the clay particles were absorbed by suspension centrifugation to settle.After centrifugation, the samples were dried at a temperature below 60 °C.The dried samples were ground with an agate mortar until the hands had no granular feeling, and then wrapped in paper for XRD determination.The X-ray diffractometer parameters were set as follows: the radiation source was CuKα, the scanning speed was set to 2° min −1 , the sampling step width was 0.02°, and the scanning range was 5°-45°.The diffraction data was interpreted based on the X-ray diffraction pattern of soil samples, and then compared with known standard minerals data to identify the minerals types present in the soil.The percentage of minerals in the soil was calculated by using the integral intensity of diffraction peaks on the diffraction pattern and the reference intensity of common minerals 33 .

Chemical methods
A glass electrode was used to measure the soil pH in a 1:2.5 soil/water suspension ratio.Soil organic carbon (SOC) content was determined using the dichromate-wet combustion method, and the C/N ratio was calculated as the ratio of SOC to total nitrogen (TN) content (determined by the Kjeldahl method).Cation exchange capacity (CEC) was determined using the Na saturation method.Each sample (0.5 g) was ground to 100 mesh (0.15 mm) in an agate mortar and formed into a tablet to measure the geochemical elements of the test soil using X-ray fluorescence spectrometry 32,34 .These geochemical elements include macroelements, trace elements, rare-earth elements, and radioactive elements.The content and composition characteristics of macroelements are widely used as indicators.Therefore, the geochemical elements in this study refer to the 10 macroelements 35 .
The chemical index of alteration (CIA) is the measure of the degree of the feldspar-to-clay conversion, which is proportional to the clay mineral/feldspar ratio.During chemical weathering, alkali metals are leached out from feldspar in the form of ions, leading to the formation of clay minerals and simultaneous fluctuation in the molar content of Al 2 O 3 .Thus, CIA is defined as follows 36 , Table 1.The information of sampling points.The average annual rainfall of the sampling points was calculated by the annual precipitation data of 1 km resolution in China (2001-2020) (National Earth System Science Data Center, National Science & Technology Infrastructure of China (http:// www.geoda ta.cn)); The average annual temperature was calculated by the monthly mean air temperature raster data of China from 2001 to 2020 (1 km resolution) 31 .The chemical index of weathering (CIW) is conceptually and computationally similar to CIA, except for the lack of K 2 O.Many scholars have found that the content of K in a sedimentary area is higher than that in provenance rock area, probably due to K metasomatism or the addition of eolian dust undergoing diagenetic processes.To eliminate the interference of diagenetic K, the concept of the CIW was proposed by Holail and Moghazi 38 , where the oxides are given in molar faction and CaO* is calculated by the same method of the CIA.Chemical weathering increase is indicated by high CIW values.The Na/K molar ratio indicates the extent of chemical weathering of plagioclase.Plagioclase is rich in Na, especially compared to potassium feldspar, which is rich in K. Plagioclase undergoes a higher rate weathering compared to potassium feldspar 10,35 , resulting in an inverse relationship between chemical weathering intensity and the Na/K molar ratio 39 .
The migration coefficient of element X (MC X ) represents the migration and enrichment characteristics of soil profile relative to bedrock during pedogenesis [40][41][42] .Titanium (Ti) is commonly used as a stationary component to determine the movement of other elements 40 , enabling for the inference of soil development degree and rate.The MC X can be calculated by the following formula, where soil and rock subscripts in the formula represent the concentrations of elements X and Ti in soil and bedrock, respectively.A MC X > 0 indicates that element X is enriched in the soil horizon relative to the parent rock and Ti, while a MC X < 0 indicates X is leaching or migration in the soil horizon.

Statistical analysis methods
One-way ANOVA and a mean comparison according to the LSD (p < 0.05) were used to evaluate the differences in physicochemical properties among landscape slope positions and soil profile horizons.Statistical analyses were conducted using SPSS 19.0.The distribution map of the sampling sites was drafted using ArcGIS 10.8, and the other diagrams were created using Excel 2016 and Origin 9.3.

Soil morphology
As shown in Table 2, the profiles with the pattern of A-C horizons were mainly concentrated at the summit and shoulder of the hillslope, the profile at the backslope and footslope was the A-B-C horizon, and the toeslope was the A-B horizon within the excavation depth.From the summit to the toeslope, the soil thickness increased significantly with the change in the soil profile configuration from 16.50 to 93.60 cm (p = 0.000).In the study area, there were three types of soil colour hues, including 10R, 2.5YR, and 5YR.The soil texture of horizon C was loam, whereas that of horizon B was mostly loam and clay loam.The soil texture of horizon A was loam to clay loam, from the summit to the toeslope (Table 3).The main soil structure of each horizon was blocky or/and granular, and the organic matter accumulation in the surface horizon, combined with mechanical ploughing, loosened the soil.The presence of a granular structure in the soil, typically characterized by loose porosity and good permeability, was generally beneficial for improving soil structure.The cohesiveness of the soil in horizon A was non-sticky, slightly sticky, and sticky from the summit to the toeslope.The cohesiveness of horizon B was mostly sticky, while that of horizon C was mostly non-sticky.Due to human cultivation or mechanical tillage, there was a small amount of intrusive material in the soil, mainly consisting of brick, tile debris, and a small amount of coal cinder.The parent rock of mudstone soil was sedimentary rock, most lacustrine facies sedimentary rock deposited during the Jurassic and Cretaceous periods, resulting in the presence of a small number of shells in the soil.

Soil physical and chemical properties
The bulk density and porosity of the soil profiles at different slope positions were showed in Table 3.The bulk density of horizons A and B of the soil profiles gradually increased from the summit to the toeslope.Additionally, the bulk density of horizon A was found to be lower than that of horizon B at all slope positions.The porosity in horizons A and B of the soil profiles also decreased gradually from the summit to the toeslope.The porosity of horizon A was higher than that of horizon B. This was primarily due to clays and oxides (such as iron and aluminum oxides) were leached downward and accumulated in the horizon B under the action of percolating water, leading to a decrease in porosity.Additionally, the topsoil was typically rich in organic matter and had higher porosity compared to the subsoil.As indicated in Table 3, the content of sand and silt fractions in horizons A and B decreased gradually from the summit to the toeslope.Conversely, the clays fraction content in horizons A and B increased gradually from the summit to the toeslope.
As shown in Fig. 3, the pH of the different horizons showed an order of C > B > A, and with a decreasing trend from the summit to the toeslope.The SOC content in horizon C was low, with no significant difference among the different slope positions (p = 0.978).The SOC content in horizons A and B gradually increased with decreasing slope elevation, and the difference in SOC above and below the backslope was significant (p = 0.000).The accumulation of SOC in horizon A at the footslope and toeslope was mainly due to soil erosion, cultivation and fertilisation, while the accumulation of SOC in horizon B was mainly caused by detachment, transportation, accumulation, and burial of deep soil in the higher topography.The hilly terrain was conducive to the accumulation of SOC content at the footslope and toeslope.Part of the P in the soil came from the parent rock, whereas  www.nature.com/scientificreports/ the other part came from the application of chemical fertilisers and plant decomposition.Although phosphorus was generally considered to be relatively immobile in the soil, it could also be redistributed through the erosion of phosphorus-containing soil particles, dissolution into runoff and migration along the slope, and leaching into the ground.Therefore, the variation in phosphorus content was observed across different slope positions and profile levels during the soil formation process (Fig. 3).Total phosphorus (TP) in the soil at the summit of the slope was significantly lower than that in the toeslope (p = 0.043).The TP in the soil above the backslope was in the order of horizon B > C > A, whereas that in the footslope and toeslope was A > B > C. The TN content from the summit to the toeslope increased with decreasing slope elevation, and there was a significant difference above and below the backslope (p = 0.000), indicating that there was TN accumulation at the foot and toeslope.In general, the TN above the backslope was in the order of horizon A > B > C of the soil profile, whereas that of the footslope and toeslope was horizon B > A > C (Fig. 4).The total potassium (TK) content from the summit to the toeslope gradually increased with decreasing slope elevation.There was a significant difference between the summit, shoulder of the slope and footslope, and toeslope, indicating that K in the soil accumulated at a lower topographic position (p = 0.000).According to the different horizons, the TK in the soil above the backslope was in the order of horizon C > B > A, whereas that at the footslope was horizon A > C > B (Fig. 3).There were differences in the CEC content among the different soil horizons.The average CEC contents in horizons A, B, and C were 27.37, 29.67, and 28.36 cmol(+) kg −1 , respectively.These results indicated that cations migrated and leached in horizon A, accumulating and becoming enriched in horizon B. The CEC above the backslope followed the order of horizon C > B > A, while below the backslope it was B > A > C.
As shown in Table 4, a noticeable negative correlation was found between sand content and SOC, TN, TK, CEC, and bulk density, and a significant positive correlation was observed between sand content and pH and porosity (p < 0.05).There was a significant negative correlation between silt content and the levels of SOC, TN, TP, TK, and CEC, as well as a significant positive correlation between silt content and pH (p < 0.05).Additionally, there was a negative correlation between clay content and pH and porosity, but a significant positive correlation between clay content and SOC, TN, TK, CEC, and bulk density (p < 0.05).

Geochemical composition and chemical weathering indices
In general, the footslope and toeslope of the hillslope showed accumulation of Al, Fe, Mg, and K, and leaching loss of Ca and Na compared with the summit and shoulder of the hillslope.There was a leaching loss of K in horizon A at the summit of the hillslope, while the variation in elements in each horizon at the shoulder of the hillslope was not obvious (Table 5).
As demonstrated in Table 5, Al, Fe, and Mg accumulated in horizon B, whereas the percentages of Ca and Na in horizon B were lower compared to horizons A and C. Additionally, Si exhibited a trend of A > B > C across various soil profile horizons.Generally, weathering tended to be most intense nearest the soil surface.Part of the weathering products formed in horizon A were leached downward under the action of percolating water, coupled with the weathering materials of horizon B itself, which led to the accumulation of Al, Fe and Mg in horizon B. Simultaneously, minerals with low weathering stability were gradually destroyed and reduced, while quartz with high weathering stability was enriched.This resulted in a sequential decrease in the percentage of Si from topsoil to subsoil.The leaching of Na and Ca was significant in horizons A and B (p < 0.05), whereas the leaching of Ca and Na was obvious in horizon B relative to horizon A in the hillslope.During the development of silicate soils, Na and Ca were leached first, followed by K, and Al and Fe were relatively enriched.The soil in this study was in the stage of leaching of Ca and Na, the leaching of soil geochemical elements was not strong, www.nature.com/scientificreports/and the degree of soil development was weak.The effect of microtopography on soil chemical weathering was greater than that on the profile.The chemical index of alteration (CIA) exhibited an increasing trend, whereas Na/K demonstrated a converse trend from summit to toeslope (Fig. 4a).The CIA values of summit, shoulder, backslope, footslope, and toeslope were 71.78, 73.17, 72.51, 74.55, and 74.55, respectively (Fig. 4b).The CIA values of horizons A, B, and C were 72.92, 74.37, and 72.61, respectively (Fig. 4c).There were significant differences in CIA values between horizons A and B, B and C (p = 0.000).The variation trend of chemical index of weathering (CIW) was basically the same as that of CIA.Furthermore, both CIA and CIW have similar indicators for reflecting the degree of soil weathering.The variance F value of the soil chemical weathering index under different factors demonstrated that the CIA, CIW, and Na/K of soil varied significantly between different horizons at various slope positions.Additionally, there was a significant interaction between slope position and horizon (Table 6).Simultaneously, the CIA, CIW, and Na/K values indicated that the soil in the study area had moderate chemical weathering, and the chemical weathering of the soil parent horizon remained at the same level from the summit to the toeslope of the hillslope.The CIA, CIW, and Na/K values of horizons A and B showed a trend of initially increasing and then decreasing with decreasing slope elevation.Horizon B at the footslope exhibited the highest degree of development, which indicated that the eroded and weathered material stripped and transported from higher landscape positions was deposited into horizon B at the footslope duo to topographical influences.
As shown in Fig. 5, the mobility of the elements varied at different landscape positions.The migration directions of Ca and Na at the summit and backslope were completely opposite to that at the footslope.The average migration coefficients of Ca at the summit, backslope, and footslope were 1.43%, − 4.93%, and − 33.43%, respectively.The average migration coefficients of Na were 16.15%, 1.73%, and − 18.43%, respectively, and the migration direction changed from enrichment to leaching.Al, Fe, and Mg were first leached and then enriched from the summit to the footslope, which may be due to the higher sand content in the soil and relatively large intergranular pores, resulting in the loss of elements under the action of rainfall and underground runoff.However, in areas with relatively low terrain, like the footslope, the abundant water conditions provided the ideal environment for soil chemical weathering.This led to the leaching of Ca and Na, while Al and Fe became enriched.

Mineralogy characteristics
The mineral composition of the soils was analysed using X-ray diffraction (Fig. 6).There was an evident mineralogical similarity between the soils at different slope positions and horizons.The mudstone soils in this study were primarily composed of illite, kaolinite, and an illite/smectite mixed horizon.The mineral composition of  the soils closely resembled that of their parent rock.These results indicated that the clay minerals in the soils originated mostly from the parent material and that they were only slightly influenced by soil-forming processes.

Effect of microtopography on the physical and chemical weathering of the soil at different slope positions
The spatial heterogeneity in particle size distribution, pH, SOC, elements, and CEC across landscape positions were influenced by microtopography 43,44 .A study conducted on the Moody and Nora soil system toposequences formed in calcareous loess in eastern Nebraske showed that sand, silt, and pH exhibited an overall increasing trend from the top to the bottom of the slope.Inversely, clay particles, organic matter (excluding toeslope), and CEC showed a decreasing trend 43 .This is contrary to our findings.The differences in particle size distribution among landscape positions may be attributed to soil erosion and the downward migration of clay particles under the action of percolating water 10 .In the present study, the texture of horizon A primarily ranged from loam to clay loam.On one hand, due to the influence of surface runoff, partial clay particles migrated along the slope and accumulate at the bottom [45][46][47] .On the other hand, under the action of percolating water, the smallest clay particles were leaching downward into the underlying horizons, resulting in an increase in the percentage of clay in horizon B (Table 3).Zhong et al. 48also showed that the sand content and mineral sand of 0.25-2 mm in the soil, developed by shale and mudstone, decreased from the top to the foot of the slope.In contrast, the clay content and aggregate content of 0.25-2 mm increased.The opposite trend in pH observed may be attributed to the topsoil analyzed by Brubaker et al. 43 being rich in Ca, and the Mg content increased continuously from the summit to toeslope.Although some leaching occurs during the downslope migration, a considerable portion still migrates to the bottom of the slope, resulting in enrichment.The decrease in organic matter from summit to footslope may also contribute to the increase in pH.In this study, the continuous enrichment of Al, leaching of Ca and accumulation of organic matter from the summit to toeslope resulted in the decrease of soil pH.This was further illustrated by the significant negative correlation between pH and aluminum oxide and SOC in Table 4.
The increase in SOC from the summit to the toeslope may be related to soil erosion.A study of toposequences in olive grove in the Mediterranean also indicated that erosion caused the transfer of organic matter from the summit to the toeslope 44 .Ouyang et al. 17 conducted a study on the CEC value of soils developed in granite, slate, limestone, sandstone and quaternary red clay in mid-subtropical Hunan Province, China.This study demonstrated that the CEC value was higher on the footslope compared to the backslope.The findings of the present study corroborate these results, showing that the CEC values on the footslope and toeslope were higher compared to other landscape positions.This higher CEC value was attributed to the accumulation of SOC and clay particles, as indicated by a significant positive correlation in Table 4.
The degree of soil chemical weathering varied with changes in the slope position and soil depth.Topographic changes caused differences in chemical weathering of the soil profile, mainly because the soil at the top and shoulder of the slope was mostly developed under natural conditions and rarely affected by human beings.In these areas, soil weathering mainly depended on the changes in natural climate conditions and the effect of soil organisms.Conversely, at the footslope and other relatively low topographic locations, soil moisture is retained, providing ideal moisture conditions for chemical weathering to occur.Meanwhile, the soil on the footslope and toeslope was affected by cultivation practices.Irrigation alters the soil moisture status, tillage changes soil aeration conditions 49 , and fertilisation increases soil nutrient elements.These factors contribute to soil mineral weathering and nutrient leaching, while also intensifying soil erosion [50][51][52] .Therefore, soil erosion caused by tillage promotes chemical weathering in the soil.

The redistribution of flow and materials by microtopography resulted in the difference of pedogenic characteristics at different slope positions
Water runs through the entire process of soil occurrence and development.It plays a crucial role in soil formation by serving as the primary medium for transporting solids and ions within the soil 53 .Microtopography dominates surface hydrological processes and affects water redistribution 54,55 , leading to variations in the physicochemical properties of soil at different slope positions 15 .In low-lying areas, the regolith is typically more extensively weathered, and the soil profile is developed with greater intensity due to the accumulation of flowing water.This water saturation also restricts drainage and ventilation, limiting the weathering of certain minerals and the decomposition of organic matter 10 .Therefore, even within a microdomain of tens of meters, the soil of the same genus also forms different textures owing to the various properties.In this study, the physicochemical properties of the soil varied with the change in the slope position, particularly between the soil above and below the backslope, with differences also seen among different horizons.The mudstone parent rock, exposed at the summit of the hilly mountainous region caused by erosion, is rich in clay minerals and has a strong water absorption capacity.Therefore, it is easily weathered physically under the influence of moist heat expansion.Consequently, stony subsoil is frequently formed at or near the summit of the slope 48 .Obi et al. 46 showed that sand content is affected by rainfall and infiltration.Excessive rainfall, when surpassing infiltration, leads to the redistribution of sand within the slope and soil horizon.This particular occurrence is commonly witnessed during thunderstorm weather conditions, subsequently compromising the stability of the soil surface on the slope 56,57 .Additionally, the study findings indicate that the level of chemical weathering in horizon A at the footslope and toeslope was comparatively lower than that of horizon B. This disparity can be attributed to the redistribution of soil materials brought about by microtopography 3 .This finding contradicts the results of other toposequences, which indicated that horizon A (or topsoil) undergoes greater chemical weathering than horizon B (or subsoil) 21,58 .This could be due to severe soil and water loss in hilly mountainous regions, where materials transported by upper erosion are deposited at the footslope and toeslope.Long-term contact between water and sediment leads to further chemical action, resulting in soil with a high organic matter content and fine texture.This soil is then buried by a new round of denudation accumulation and self-weathered soil, eventually becoming a B-horizon with a higher degree of development than the topsoil.

Conclusion
In this study, we investigated the influence of microtopography on the morphological characteristics, physiochemistry, and geochemical attributes of the profiles.The results indicated that the morphological characteristics of the mudstone soil profile were mainly inherited and affected by the parent material.From the summit to the toeslope, the profile configuration of the mudstone soil changed from A-C to A-B-C, and the thickness of the soil increased significantly.The bulk density, clay fraction, soil organic matter, TN, TP, TK, and CEC increased

Figure 4 .
Figure 4.Chemical weathering parameters of soil profiles (a) and their means (b) at different landscape positions, and means at the A, B, and C horizons (c).

Figure 5 .
Figure 5. Geochemical elements migration coefficient of soil profiles at different slope positions.

Figure 6 .
Figure 6.X-ray diffraction data for soils at different slope positions (A-horizon A, B-horizon B, and Chorizon C).The sequence of treatments is represented by three different colours: the black (top), red (middle position), and blue (bottom) curves represent the air-dried mount (AD) samples, samples heated at 550 °C (K550), and glycol solvated (EG) samples, respectively.I: illite; K: kaolinite; I/S: illite/smectite mixed-horizon.

Profile No. Horizon Depth (cm) Soil colour Soil structure Plasticity Animal activity Intrusions Dry state Wet state
37ntinuedwhere all variables represent the molar faction of macroelement oxides and CaO* is the molar faction of CaO associated with silicate minerals calculated using the McLennan method37.Generally, CIA values are below 50 for unweathering with feldspar, 50-65 for weak weathering with plagioclase, 65-85 for moderate weathering with illite and montmorillonite, and near 100 for strong weathering with kaolinite and chlorite.Additionally, CIA indirectly indicates the change in climate, with higher CIA values reflecting warmer and wetter climates.

Table 2 .
Morphological attributes of the soil profiles.BS, blocky structure; GS, granular structure.

Table 3 .
The bulk density, porosity, and particle size distribution of soil profiles at different slope positions.

Table 5 .
Geochemical element contents of soil profiles at different positions.Values for different geochemical element contents in a column followed by the same lowercase letter are not significantly different at p < 0.05.Data are means ± SD.

Table 6 .
Analysis of variance F value of chemical weathering indicators of soils at different slope positions and horizons.S × H, interaction of slope position and horizon.